1. Field of the Invention
This invention relates to a process for preparing a carbon electrode, and more particularly to a process for preparing a carbon electrode including lithium as a dopant material.
2. Description of the Related Art
Active research has been conducted in order to utilize graphite as an electrode for rechargeable batteries, since it has a layered structure and forms interlayer compounds with various materials. Especially, a graphite electrode as a negative electrode for secondary lithium batteries utilizing an interlayer compound formed with lithium as a dopant is expected to realize a high capacity negative electrode free from dendrite (for example, as disclosed in Japanese Published Unexamined Patent Application No. 208079/1982).
Generally, the following techniques for forming a graphitized interlayer compound by intercalating lithium into natural graphite are known hitherto.
(i) Electrochemical process, and PA1 (ii) Vapor phase methods directly contacting lithium vapor with graphites
Lithium is, however, not intercalated electrochemically into natural graphite in a non-aqueous electrolyte due to the cause of undesirable side reaction, so that a natural graphite cannot be put into practical use as a negative electrode for secondary lithium batteries utilizing a non-aqueous electrolyte.
Also, the so-called HOPG (Highly-Oriented Pyrolytic Graphite), a high temperature pyrolytic carbon is also used as a material for an electrode in place of natural graphite The HOPG is highly crystalline, so that it is hard to intercalate lithium in accordance with the electrochemical techniques, while a technique for intercalating lithium by the vapor phase process has been practically proposed ("Aspects of alkali metal intercalation and deintercalation in highly oriented pyrolytic graphites" Synthetic Metals, 3(1981) pp. 27-39).
However, when the Li-intercalated HOPG provided by the gas-phase process is used as a negative electrode for lithium batteries utilizing a non-aqueous electrolyte, the electrode itself is disintegrated upon discharge. Hence, it cannot be put into practical use.
In this regard, the inventors of the present invention have already discovered the fact that when a graphitized pyrolytic carbon formed by a low temperature vapor phase pyrolysis of hydrocarbon is used in place of the above natural graphite, it allows the electrochemical intercalation of lithium into the above pyrolytic carbon in non-aqueous electrolyte, and the above pyrolytic carbon is excellent as a negative electrode for secondary lithium batteries (Japanese Published Unexamined Patent Application No. 24555/1988). Furthermore, it has been discovered that the above pyrolytic carbon when deposited on a substrate of an iron family metal exhibits flat and excellent discharge properties (Japanese Published Unexamined Patent Applications Nos. 245855/1988 and 102167/1988). The pyrolytic carbon exhibits good properties in charge and discharge test cycles carried out in a non-aqueous electrolyte.
However, a discharge capacity (mAh/g) of the above pyrolytic carbon negative electrode is smaller than a value calculated from a composition C.sub.6 Li of an ideal layered compound formed with lithium and carbon and thereby not satisfactory for practical use.
To be noted is that intercalation of lithium into the pyrolytic carbon in accordance with the vapor phase techniques has not at all been put in use, and the intercalation techniques has been considered to cause the same problem as aforesaid.